LM3914 Datasheet, PDF (15/19 Pages) National Semiconductor (TI) – Dot/Bar Display Driver

Application Hints (Continued)
APPLICATION TIPS FOR THE LM3914 ADJUSTABLE
REFERENCE
GREATLY EXPANDED SCALE (BAR MODE ONLY)
Placing the LM3914 internal resistor divider in parallel with a
section (≅230Ω) of a stable, low resistance divider greatly
reduces voltage changes due to IC resistor value changes
with temperature. Voltage V₁ should be trimmed to 1.1V first
by use of R2. Then the voltage V₂ across the IC divider string
can be adjusted to 200 mV, using R5 without affecting V₁.
LED current will be approximately 10 mA.
NON-INTERACTING ADJUSTMENTS FOR EXPANDED
SCALE METER (4.5V to 5V, Bar or Dot Mode)
This arrangement allows independent adjustment of LED
brightness regardless of meter span and zero adjustments.
First, V₁ is adjusted to 5V, using R2. Then the span (voltage
across R4) can be adjusted to exactly 0.5V using R6 without
affecting the previous adjustment.
R9 programs LED currents within a range of 2.2 mA to 20 mA
after the above settings are made.
Greatly Expanded Scale (Bar Mode Only)
DS007970-15
ADJUSTING LINEARITY OF SEVERAL STACKED
DIVIDERS
Three internal voltage dividers are shown connected in se-
ries to provide a 30-step display. If the resulting analog meter
is to be accurate and linear the voltage on each divider must
be adjusted, preferably without affecting any other adjust-
ments. To do this, adjust R2 first, so that the voltage across
R5 is exactly 1V. Then the voltages across R3 and R4 can
be independently adjusted by shunting each with selected
resistors of 6 kΩ or higher resistance. This is possible be-
cause the reference of LM3914 No. 3 is acting as a constant
current source.
The references associated with LM3914s No. 1 and No. 2
should have their Ref Adj pins (pin 8) wired to ground, and
their Ref Outputs loaded by a 620Ω resistor to ground. This
makes available similar 20 mA current outputs to all the
LEDs in the system.
If an independent LED brightness control is desired (as in
the previous application), a unity gain buffer, such as the
LM310, should be placed between pin 7 and R1, similar to
the previous application.
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